The use of ophthalmic lenses for the correction of ametropia is well known. For example, multifocal lenses, such as progressive addition lenses ("PALs"), are used for the treatment of presbyopia. Most conventional PALs are produced by placing progressively changing radii of curvature on the convex surface of the lens to provide base curves corresponding to distance, intermediate, and near vision power zones. Typically, distance power, cylindrical power for correction of the lens wearer's astigmatism, and the axis of the cylinder are provided for on the concave surface of the lens.
PALs are appealing to the wearer because the lenses are free of the visible ledges between the zones of differing optical power that are found in other types of multifocal lenses, such as bifocals and trifocals. However, an inherent disadvantage of PALs is unwanted astigmatism, or astigmatism introduced or caused by one or more of the lens surfaces. Generally, the unwanted astigmatism is located on either side of the near and intermediate vision zones of the lens and reaches a localized maximum that corresponds approximately to the near vision dioptric add power of the lens.
Any number of lens designs have been tried in attempting to reduce unwanted astigmatism. However, although the state-of-the-art PAL designs provide some minimal decrease in unwanted astigmatism, large areas in the lenses' peripheries remain unusable due to the astigmatism. Thus, a need still exists for a PAL that reduces unwanted astigmatism.